Nitrogen fixation (acetylene reduction) inAquaspirillum magnetotacticum

Aquaspirillum magnetotacticum strain MS-1 and two nonmagnetic mutants derived from it reduced C₂H₂ microaerobically but not anaerobically even with NO₃ ^?. This organism apparently is not capable of NO₃ ^?-dependent nitrogen fixation. Cells ofA. magnetotacticum reduced C₂H₂ at rates comparable to those ofAzospirillum lipoferum grown under similar conditions, but much lower than that ofAzotobacter vinelandii grown aerobically. Cells ofA. magnetotacticum in anaerobic cultures lacking NO₃ ^? did not reduce C₂H₂ until O₂ was introduced. Optimum rates of C₂H₂ reduction byA. magnetotacticum were obtained at 200 Pa O₂. C₂H₂ reduction was inhibited by more than 1 kPa O₂ or 0.2 mM NO₃ ^? or NH₄ ⁺. These results suggest thatA. magnetotacticum fixes N₂ only under microaerobic, N-limited conditions.